1. Field of the Invention
The present invention relates to a refractive ophthalmic lens having a reduced thickness, the lens having at least one of its surfaces formed with a number of concentric circular bands, the bands having an elliptical cross-section.
2. Description of the Prior Art
One form of multifocal ophthalmic lens is proposed in U.S. Pat. Nos. 4,210,391; 4,340,283 and 4,338,005 (Allen L. Cohen) whereby a multifocal Fresnel lens is constructed by means of modifying the phase separating annular rings of a zone plate, with curved or inclined optical facets of varying refractive indices which then function as Fresnel rings corresponding to the different focal powers desired. To counteract the inherent problems of a Fresnel lens with small annular zone widths where optical aberrations are introduced by diffraction effects, a zone plate is introduced. Thus, an ophthalmic lens according to the above patents would be a composite device comprised of a Fresnel lens and a zone plate.
A zone plate, essentially, is a diffraction optical device that consists of a series of concentric opaque rings of such predetermined width that rays from alternate half period elements are cut off. Such a device has some properties of a converging lens. Therefore, it has been attempted to use the combination of Fresnel lens and zone plate to approximate the function of a true refractive lens in either bifocal or multifocal configurations.
Anticipating the limitations presented by such a design, two of which are limited image brightness and limited dioptric power, another form of a multifocal ophthalmic lens has been proposed. Here the lens is also a composite device as described above except that a phase shift multifocal zone plate is constructed in such a way that some of the zone plate focii actually coincide with some of the multifocal Fresnel lens focii. This is obviously done to increase image brightness at each of the focii. An ophthalmic lens designed on this basis will present two major drawbacks, both of which cannot be circumvented due to the inherent optical properties of such a device.
The first drawback is the fundamental problem of inadequate image brightness typical to such designs, especially considering the wide range of focal points in such a lens. The other drawback is the limited range of focal lengths achievable with such a device.
Specifically, if a bifocal lens of a certain power is constructed, a very limited additional power for near vision can be provided. Whenever an appreciable additional power is required, the lens would be of very little value. The physical properties of light, such as wavelength and relative intensity, will fundamentally limit the performance of any multifocal ophthalmic lens, such as those proposed in U.S. Pat. Nos. 4,210,391; 4,340,283 and 4,338,005 mentioned earlier, especially when a small size is an absolute necessity, as is the case with contact or intraocular lenses.
It is paramount to keep in mind that any diffractive or composite diffractive lenses or devices are principally different from refractive lenses and only approximate a true refractive system.
Another type of an ophthalmic lens has been proposed in U.S. Pat. No. 4,418,991 (Joseph L. Breger). More specifically, a contact lens that would provide a distance correction at the center, while increasing the diopter adds away from the center would provide for intermediate and close viewing. (See Col. 5 lines 1 through 21 of Breger).
The dioptric power change in the above lens is achieved by progressively changing the radius of curvature of the posterior surface. A major limitation of such a lens is that the focal planes provided are not discrete but a progressive succession of innumerable possibilities, and therefore no truly sharp focal planes may be provided.
Another drawback is that the images produced will be located on substantially different areas of the retina which as is commonly known do not have the same sensitivity. Still another factor limiting the performance of such a lens will be its absolute dependence on the position relative to the pupil and the pupil size as well as its dilation. It is common knowledge that to achieve a continuously ideal position with a contact lens is not frequently possible. To control the pupil size or dilation relative to changing luminosity is even more difficult. It is important to note that lenses of progressive power change designs share common drawbacks and limitations regardless of whether the distance vision is in the center or toward the edges of the viewing area. Obviously the limiting factors are not equivalent but their presence severely curtails the performance of such or similar lenses in one way or another.
Still another design of a multifocal ophthalmic lens is proposed in French Pat. No. 1,319,800 by Sohnges. The lens in question would have discrete dioptric powers provided by concentric circular zones. The preferred version has the distance vision portion in the center of the lens and increasing dioptric power toward the periphery to provide intermediate and near vision. Although the formed images will be clearer than in progressive power increase or decrease designs, the performance of the lens in question will be limited due to a critical dependence on pupil size, centration of the lens relative to the pupil, ambient illumination as well as due to the creation of images on substantially different areas of the retina. Essentially the drawbacks of this design are similar to the ones in the progressive power change lenses differing mainly in the fact that the powers provided are discrete and not continuous.
Another form of an ophthalmic lens is described in W088/09950 (Valdemar Portney). The proposed lens has a plurality of concentric alternating zones with a continuously varying power within each zone as well as in transition from one zone to another. In one version, continuous alternating power is accomplished by a continuously changing curvature of the posterior surface of the lens. In another version continuous, alternating power variation is accomplished by creating non-homogeneous surface characteristics having refractive material indices which continuously vary in the lens radial direction. In other words, the optical portion of such a lens is comprised of a number of concentric zones. The distinctive characteristic of this design (page 9, lines 2 through 5) is that each zone is considered to include a complete cycle of powers from intermediate to high to intermediate to low, then back to intermediate.
Still another type of multifocal ophthalmic lens is described in U.S. Pat. No. 4,798,608 (Dennis T. Grendahl). The invention pertains to an implantable intra-ocular lens containing a laminated structure comprising a number of laminated planar or curved elements. The incident rays are brought to a focus on a portion of the retina and are dependent on the number of lens elements traversed by a ray. Areas of differing powers are provided by forming a uniform lens surface over a composite laminated structure of laminate elements having different indices of refraction. Although the field of the invention claims to encompass contact lenses, producing a contact lens according to this invention would be highly impractical, given the physical structure of a contact lens. A typical thickness of an average rigid contact lens is about 0.12 mm across the cross section. Assuming only a three layer laminate structure according to the invention, to provide near, intermediate and distant vision, the curvatures in question relative to indices of refraction to provide dioptric requirements of a typical contact lens will render the center thickness of such a lens in the range of 1.00 to 1.30 mm, which is not practical.
A related invention is described in U.S. Pat. No. 4,795,462 (Dennis T. Grendahl). It pertains mostly to intraocular lenses but also covers contact and intracorneal lenses. A lens according to Grendahl contains annular elements each of which serves to bring the impinging rays from an object at a predetermined distance to a focus on a particular region of the retina. The lens is a composite of a cylindrical and annular optical lens elements each of which has a distinct power and focal length (see Col. 1, line 65 thereof). It differs from many other annular designs by the fact that it has a cylindrically segmented composite zone of focus.
A different multifocal lens design is proposed in U.S. Pat. No. 4,704,016 (John T. deCarle). A contact lens is described, wherein the major viewing area is divided into a number of near and distant vision zones. Of relevance to the present discussion is the case of a lens (see Col. 2, line 13), the front or the back surface of which is formed with a series of concentric areas, each annular area being cut alternately for distant and near vision. A lens produced according to that invention (see Col. 2, line 67 through Col. 3, line 16) will, as it is readily understood, have sharp steps at the lines of transition between annular areas. The magnitude of these steps for a typical contact lens would be in the order of 140 microns or 0.014 mm. This in itself will produce aberrations in the form of a multitude of prisms as well as diffractive effects. These combined (or even separately) will render the design disadvantageous. Therefore, a way of circumventing sharp steps is proposed (see Col. 3, line 4 through 16), whereby the center of curvature continually changes position relative to the central axis (which happens to be the optical axis as well) when moving the cutting tool to produce a profile which would have no sharp steps. In this case, annular rings of like power will have the same curvature but different centers both geometrically and optically. This means that each of these centers is located on a different optical axis. None of this multitude of optical axes coincide with the central principal optical axis of the lens (and the eye) except for the central zone axis and the zone immediately adjacent to it. Needless to say that a lens having a multitude of optical axes, once placed on an eye which has only one visual axis, will present a series of blurred images. This is of little value in view of the vision correction requirements.
A continuation of the above patent is U.S. Pat. No. 4,890,913. Here, as in the preceding case, a contact lens is formed, the viewing area of which is comprised of a plurality of annular viewing zones, each near vision zone being adjacent to a distant vision zone. The drawbacks of this lens will be exactly the same as in the preceding case, since it is essentially the same type of contact lens.
In retrospect, all of the relevant prior art examples have the common characteristic of circular (annular) concentric elements formed in the viewing area of the lens. All of them have drawbacks when considered in contact lens configuration.
One major common drawback of contact lenses of all types is a rapid increase in lens thickness with an increase in prescribed refractive power of a lens. This is true for both converging (positive) and diverging (negative) lenses. Obviously, depending on the refractive type of the lens the said increase in thickness manifests itself in different portions of the lens. Converging lenses exhibit the maximum thickness at the centre of the lens whereas diverging lenses exhibit an increase in thickness reaching a maximum at the outer periphery of the lens.
One traditional way of producing lenses of reduced thickness is lenticulation. Essentially, lenticulation is a method of reduction of lens thickness by confining the optical portion of the lens within a central area called the optical zone. This in effect reduces the diameter of the refractive element which therefore allows a reduction in thickness. The final diameter of the optical zone is dictated by the pupil diameter, lens position and displacement during blink cycle. The overall lens diameter is determined by the anatomical factors of the eye as well as by mechanical properties of the lens material and is greater than the optical zone. The area concentric to the said optical zone and extending to the overall lens diameter is called peripheral zone also referred to as lenticular carrier. This carrier is relatively thin when compared to the optical zone usually reaching minimum thickness at the edge of the lens.
There is, however, a requirement for a smooth junction between the optical zone and the peripheral zone. This requirement inevitably compromises the thickness of the said peripheral carrier such as in the case of diverging lenses where the junction between the optical and peripheral portions represents the thickest part of the lens.